The present invention relates to an electronic image sensing apparatus, and more particularly to an electronic still camera containing a solid state image sensor such as a charge transfer device, which is suitable for high shutter speed operation.
A solid state image sensor, such as a charge transfer device (CTD) including a charge coupled device (CCD) is superior to the conventional image pick-up tube in many points. For example, the image sensor is small in size, light in weight, and high in reliability. Further, it is essentially free from pattern distortion and sticking, and is little influenced by after image. Because of such advantageous features, the solid state image sensor has a prospect of increasing applications in many fields, for example, in ITV, and small commerical video cameras. In particular, the application of the solid state image sensor to an electronic still camera replacing a conventional camera using a silver salt film has recently attracted a great deal of attention.
The solid state image sensor for the present VTR camera is constructed so as to be adaptable to the NTSC television system in which one frame consists of two fields. For reading out and reproducing the picture information of all the photosensitive cells of the image sensor, the shutter of the electronic camera must be open for at least 1/30 of a second or more. In normal use, however, 1/30 second is a relatively long time. Unless the camera body, for example, is firmly fixed by means of a tripod or the like an undesirable movement of the camera will inevitably be caused by uncontrollable movements of the operator's body.
For obtaining a high speed photography within the shutter opening time of 1/30 second, the read frequency of the image sensor must be set at a high frequency in order to reduce one frame period. In a CCD image sensor having 500 (vertical).times.400 (horizontal) picture elements, for example, the read frequency is ordinarily high, e.g. 7.16 MHz. Even if the read frequency can be set at 14.32 MHz, i.e., double the former, the shutter speed of the electronic camera is at most 1/60 second. A shutter speed of approximately 1/1000 second is necessary for high speed photography. Therefore, if only the read frequency of the CCD image sensor is increased to obtain a shutter speed of 1/1000 second, the read frequency will be 240 MHz. However, if the read frequency of the CCD image sensor is set at such an extremely high value, the drive and the signal processing of the CCD image sensor become extremely difficult, and therefore it is not practical.
Accordingly, when designing an electronic still camera for high speed photography while allowing for the above-factors, the interlacing photography technique has not been employed. Therefore, a still image formed by the electronic camera has one field picture, and the number of picture elements is reduced. This undesirably leads to a degradation of the resolution of the picture image, and thus, poor picture quality.